This invention relates to conductive and weldable anti-corrosive compositions for coating metal surfaces and to a process for coating metal surfaces with electrically conductive organic coatings.
In the metals processing industry, in particular in automotive construction, the metallic components of the products must be protected from corrosion. According to the conventional prior art, this is achieved by initially coating the sheet metal in the rolling mill with anti-corrosive oils and optionally with drawing greases before forming and stamping. In automotive construction, sheet metal components appropriately formed for bodywork or bodywork components are stamped out and formed by deep drawing using such drawing greases or oils and then generally joined together by welding and/or crimping and/or adhesive bonding and then elaborately cleaned. Anti-corrosive surface pretreatment, such as phosphating and/or chromating, is then performed, whereupon a first lacquer layer is applied onto the components by electrocoating. Especially in the case of automotive bodywork, this first electrocoating is followed by the application of further lacquer layers.
There is a need to find simpler production processes which make it possible to weld already precoated sheet metal and to electrocoat it in a proven manner. There is thus a range of processes in which, after phosphating and/or chromating, an organic coating which is conductive to a greater or lesser degree is applied in the so-called coil coating process. These organic coatings should generally be of a composition such that they have sufficient electrical conductivity not to impair the electrical spot welding process. It should moreover be possible to coat these coatings with conventional electrocoating lacquers. It should furthermore be possible to stamp and form the sheet metal coated in this manner with reduced usage of deep drawing greases or oils. Recently, especially in the automotive industry, galvanised and/or alloy-galvanised sheet steel and aluminum and magnesium sheet have increasingly been used in addition to standard sheet steel.
It is in principle known to coat sheet steel with organic coatings which are weldable and are applied directly in the rolling mill using the so-called coil coating process.
DE-C-3412234 thus describes a non-blocking and weldable anti-corrosive primer for electrolytically thin galvanised, phosphated or chromated and formable sheet steel. This anti-corrosive primer consists of a mixture of over 60% zinc, aluminum, graphite and/or molybdenum disulfide and another anti-corrosive pigment and 33 to 35% of an organic binder, together with about 2% of a dispersion auxiliary or catalyst. Polyester resins and/or epoxy resins and derivatives thereof are proposed as the organic binder. It is assumed that this technology forms the basis of the coating composition known in the industry under the name xe2x80x9cBonazinc 2000xe2x80x9d. Although this process offers some advantages in comparison with the above-stated method (temporary corrosion protection with anti-corrosive oils, followed by subsequent degreasing once the metal components have been joined), the process described in DE-C-3412234 is still in great need of improvement:
this coating is not sufficiently spot weldable
the stoving temperature for such coatings at a peak metal temperature (PMT) of 250 to 260xc2x0 C. is still too high. Many recent steels with the xe2x80x9cbake-hardeningxe2x80x9d effect cannot be used at such high stoving temperatures.
lacquer adhesion onto the pretreated substrates, preferably galvanised steels, is still inadequate, especially if the sheets are subjected to relatively severe forming in the press.
According to the teaching of DE-C-3412234, the organic binder may consist of polyester resins and/or epoxy resins and derivatives thereof. An epoxy/phenyl precondensate, an epoxy ester and linear, oil-free terephthalic acid based copolyesters are explicitly mentioned.
EP-A-573015 describes organically-coated composite sheet steel consisting of a surface coated on one or two sides with zinc or zinc alloy, which surface is provided with a chromate film and, thereon, an organic coating having a film thickness of 0.1 to 5 xcexcm. The organic coating is formed from a primer composition consisting of an organic solvent, an epoxy resin having a molecular weight of between 500 and 10000, an aromatic polyamine and a phenol or cresol compound as accelerator. The primer composition furthermore contains a polyisocyanate and colloidal silica. According to this document, the organic coating is preferably applied to a dry film thickness of 0.6 to 1.6 xcexcm as layers thinner than 0.1 xcexcm are too thin to provide corrosion protection. Film thicknesses of above 5 xcexcm, however, impair weldability. DE-A-3640662 similarly describes surface treated sheet steel comprising sheet steel provided with coating of zinc or zinc alloy, a chromate film formed on the surface of the sheet steel and a layer of a resin composition formed on the chromate film. This resin composition consists of a basic resin, which is produced by reacting an epoxy resin with amines, together with a polyisocyanate compound. This film too may only be applied to dry film thicknesses of less than about 3.5 xcexcm as weldability is severely reduced at greater film thicknesses.
EP-A-380024 describes organic coating materials based on a bisphenol A type epoxy resin having a molecular weight of between 300 and 100000, together with a polyisocyanate or blocked polyisocyanate, pyrogenic silica and at least one organic colouring pigment. In this process too, pretreatment with chromate to form a thick Cr coating is required. The organic layer cannot be thicker than 2 xcexcm as sheets having thicker organic coats cannot satisfactorily be spot welded and the properties of the electrocoating lacquer applied onto the organic coating are degraded.
An object thus arose of providing coating compositions which satisfy the automotive industry""s requirements in all respects. In comparison with the prior art, it is intended to improve the following properties of the organic coating compositions suitable for the coil coating process:
lower stoving temperature preferably no higher than 210 to 235xc2x0 C. PMT
distinct reduction in white rust on galvanised sheet steel in the salt spray test to DIN 50021, i.e. better corrosion protection
improvement in adhesion of the organic coating on the metallic substrate assessed by the T-bend test (ECCA standard) and impact test (ECCA standard)
sufficient corrosion protection even with a thin Cr coating, preferably using Cr-free pretreatment methods
cavity sealing with wax or products containing waxes, which is still conventional, should become superfluous thanks to the improved corrosion protection
suitability for spot welding.
The solution to this problem according to the present invention is stated in the claims. The present solution essentially comprises the provision of coating compositions containing 10 to 40 wt. % of an organic binder, 0 to 15 wt. % of a silicate-based anti-corrosive pigment, 40 to 70 wt. % of powdered zinc, aluminum, graphite and/or molybdenum disulfide, together with 0 to 30 wt. % of a solvent, wherein the organic binder consists of at least one epoxy resin, at least one curing agent selected from guanidine, substituted guanidines, substituted ureas, cyclic tertiary amines and mixtures thereof, together with at least one blocked polyurethane resin.
The solution according to the present invention also comprises the use of the above-stated composition for coating sheet metal in the coil coating process.
The solution according to the present invention furthermore comprises a process for coating metal surfaces with a conductive organic anti-corrosive layer characterised by the following stages:
conventional pretreatment consisting of
cleaning
optionally phosphating
chromating
optionally chromium-free pretreatment
coating with a composition of the above-stated type to a film thickness of 1 to 10 xcexcm, preferably of between 2 and 5 xcexcm
stoving of the organic coating at temperatures of between 160xc2x0 C. and 260xc2x0 C. peak metal temperature (PMT).
The metal surfaces to be coated according to the present invention are preferably iron (sheet steel). galvanised and alloy-galvanised steels, aluminum or magnesium.
For the purposes of the present invention, an electrically conductive coating should be taken to mean one which is weldable under joining conditions conventional in the automotive industry, preferably using the spot welding process. These coatings furthermore have sufficient conductivity to ensure complete deposition of electrocoating lacquers.
The epoxy resin is an essential constituent of the organic binder of the anti-corrosive composition according to the present invention. An epoxy resin or a blend of two or more epoxy resins may be used here. The epoxy resin or resins may have a molecular weight of between 300 and 100000, with epoxy resins having at least two epoxy groups per molecule and a molecular weight of above 700 preferably being used, as experience has shown the higher molecular weight epoxies to give rise to no occupational hygiene problems during application. Numerous epoxy resins may in principle be used, such as the glycidyl ethers of bisphenol A or the glycidyl ethers of novolac resins. Examples of the first-stated type are commercially available under the trade names xe2x80x9cEPICOTE 1001xe2x80x9d, xe2x80x9cEPICOTE 1004xe2x80x9d, xe2x80x9cEPICOTE 1007xe2x80x9d and xe2x80x9cEPICOTE 1009xe2x80x9d from Shell Chemie. Numerous other conventional commercial epoxy resins of the bisphenol A glycidyl ether type may also be used, as well as the above-stated epoxy resins.
Examples of novolac epoxy resins are the xe2x80x9cARALDITExe2x80x9d ECN grades from Ciba Geigy, DEN grades from Dow Chemical and numerous other manufacturers.
Polyesters bearing epoxy groups, which also includes the epoxy derivatives of dimeric fatty acids, may also be used as an epoxy resin binder component.
These epoxy resins to be used according to the present invention are preferably solid in the solvent-free state at room temperature; during production of the composition, they are used as a solution in an organic solvent.
The curing agent or agents for the organic binder may be guanidine, substituted guanidines, substituted ureas, melamine resins, guanamine derivatives, cyclic tertiary amines, aromatic amines and mixtures thereof. The curing agents may here not only be included stoichiometrically in the curing reaction, but may also be catalytically active. Examples of substituted guanidines are methylguanidine, dimethylguanidine, trimethylguanidine, tetramethylguanidine, methylisobiguanidine, dimethylisobisguanidine, tetramethylisobiguanidine, hexamethylisobiguanidine, heptamethylisobiguanidine and cyanoguanidine. Examples of suitable melamine resins are methoxymethylmethylolmelamine, hexamethoxymethylmelamine, methoxymethylmelamine, hexamethoxymethylmelamine. Examples of suitable guanamine derivatives which may be mentioned are alkylated benzoguanamine resins, benzoguanamine resins or methoxymethyl/ethoxymethylbenzoguanamine.
Examples of catalytically active substituted ureas are in particular p-chlorophenyl-N,N-dimethylurea (Monuron) or 3,4-dichlorophenyl-N,N-dimethylurea (Diuron). Examples of catalytically active tertiary alkylamines are tris(dimethylaminomethyl)phenol, piperidine and derivatives thereof, diethanolamines and various imidazole derivatives. Representatives of the many usable imidazole derivatives which may be mentioned are: 2-ethyl-4-methylimidazole (EMI). N-butylimidazole, benzimidazole, Nxe2x80x94C1 to C12 alkylimidazoles. Further examples of tertiary amine derivatives are aminooxadiazole, tertiary amine oxides, diaza aromatic tertiary amines, such as methylpyrazines, diallyltetrahydrodipyridyl and hydrogenated pyridine bases. Less reactive diamines may furthermore be present as a curing agent component, in particular aromatic diamines, such as diaminodiphenyl sulfone, 4,4xe2x80x2-methylene dianiline, m-phenylene diamine or also polyoxyalkylene polyamines of the xe2x80x9cJEFFAMINExe2x80x9d type and similar diamines.
Blocked polyurethane resins for the purposes of the present invention are di- or poly-isocyanate compounds, which are obtained in a manner known per se by reacting aliphatic, alicyclic or aromatic isocyanates having at least 2 isocyanate groups per molecule with polyols, wherein in this first stage the isocyanate groups are used in stoichiometric excess relative to the alcohol groups. In a subsequent stage, the remaining isocyanate groups are then reacted in a known manner with blocking agents for the isocyanate groups. Examples of isocyanates which may be mentioned are: m-phenylene diisocyanate, p-phenylene diisocyanate, 2,4-tolylene diisocyanate (TDI), 2,6-tolylene diisocyanate (TDI), p-xylene diisocyanate, diphenylmethane diisocyanate (MDI), hexamethylene diisocyanate, dimeric acid diisocyanate, 1-isocyanatomethyl-3-isocyanato-1,5,5-trimethylcyclohexane (IPDI), hydrogenated MDI (H12MDI), tetramethylxylylene diisocyanate (TMXDI), the biuretisation product of hexamethylene diisocyanate, the isocyanuratisation product of hexamethylene diisocyanate and the isocyanuratisation product of IPDI.
Dihydric alcohols, such as ethylene glycol, propylene glycol, butanediol, hexanediol, and the hydroxy-functional reaction products thereof with dicarboxylic acids (polyester polyols) or the alkoxylation products thereof with ethylene oxide and/or propylene oxide or mixtures thereof (polyether polyols) may be used as the polyol. The above-stated dihydric alcohols may here be entirely or partially replaced by trihydric starter alcohols, such as glycerol or trimethylolpropane, or tetrahydric alcohols, such as pentaerythritol.
Hydroxy-functional acrylate and/or methacrylate homo- or co-polymers may also be used as the polyol component.
Any known blocking agents may be used as the blocking agent (protective group) for the isocyanate groups remaining after the reaction of the polyisocyanate with the polyol, the following being mentioned by way of example, lower aliphatic monoalcohols, such as methanol, ethanol, propanol, butanol or octyl alcohol, together with monoethers of ethylene glycol and/or diethylene glycol, aromatic hydroxy compounds, such as phenol, alkylphenols or (alkyl)cresols. Oximes, such as acetone oxime, methyl ethyl ketone oxime and the like, may also be used as a blocking agent. Lactam blocking agents which may be mentioned are xcex5-caprolacatam, while CH-acidic xcex2-dicarbonyl compounds, such as malonic esters, may also be considered as blocking agents.
Particularly preferred organic binders are those which contain a blocked polyurethane resin based on the more highly reactive aromatic polyisocyanates, in particular MDI, blended with a blocked polyurethane resin based on aliphatic polyisocyanates, in particular IDPI or TMXDI.
The anti-corrosive composition additionally contains 0 to 30 wt. % of a solvent or solvent mixture, wherein a proportion of this solvent or solvent mixture may already be introduced by the epoxy resin component or polyurethane resin component, in particular if conventional commercial binder components are used for this purpose. Suitable solvents are any solvents conventional in the lacquer industry based on ketones, such as methyl ethyl ketone, methyl isobutyl ketone, methyl n-amyl ketone, ethyl amyl ketone, acetylacetone, diacetone alcohol. Aromatic hydrocarbons, such as toluene, xylene or mixtures thereof, may also be used, as may aliphatic hydrocarbon mixtures having boiling points between about 80 and 180xc2x0 C. Further suitable solvents are, for example, esters, such as ethyl acetate, n-butyl acetate, isobutyl isobutyrate, or alkoxyalkyl acetates, such as methoxypropyl acetate or 2-ethoxyethyl acetate. Monofunctional alcohols, such as isopropyl alcohol, n-butanol, methyl isobutyl carbinol or 2-ethoxyethanol, or monoalkyl ethers of ethylene glycol, diethylene glycol or propylene glycol may also be mentioned as representatives of many suitable solvents. It may be convenient to use mixtures of the above-stated solvents.
The conductive and weldable anti-corrosive composition furthermore contains finely divided conductive extenders in quantities of between 40 and 70 wt. %. Powdered zinc, powdered aluminum, graphite and/or molybdenum disulfide, carbon black, iron phosphide or BaSO4 doped with tin or antimony may be mentioned by way of example.
0 to 15 wt. % of silicate-based anti-corrosive pigments may additionally be used. Such anti-corrosive pigments are known and zinc/calcium/aluminum/strontium polyphosphate silicate hydrate, zinc/boron/tungsten silicate, doped SiO2 may be mentioned by way of example.
Familiar known additives, such as lubricants, soluble dyes or colouring pigments, together with wetting agents and levelling auxiliaries, may also be used.
The conductive, weldable anti-corrosive compositions according to the present invention are in particular suitable for coating sheet metal using the coil coating process. To this end, the sheet metal is initially subjected to conventional pretreatment processes, such as cleaning and degreasing. Conventional phosphating processes and chromating processes may optionally subsequently be performed. A particular advantage of the anti-corrosive compositions according to the present invention is that chromium-free pretreatment processes may also be used with success.
After the pretreatment, coating with the anti-corrosive composition according to the present invention is performed using a conventional coil coating process. Film thicknesses (dry film thicknesses) of 1 to 10 xcexcm, in particular of 2 to 5 xcexcm are particularly preferred. The organic coating is stoved at temperatures of between 160xc2x0 C. and 260xc2x0 C. peak metal temperature (PMT), preferably between 180xc2x0 C. and 235xc2x0 C. PMT.